This invention relates to an electronic timepiece of the kind comprising means for producing drive pulses, a motor having a rotor and means for turning the rotor through a set angle in response to each drive pulse, an autonomous source for supplying the drive pulse producing means with electric energy, means for producing a signal upon detecting the imminent end of the source's life and means for producing a warning signal in response to the signal from the detecting means.
Electronic timepieces, in particular watches and alarm and other small clocks of the analogue kind, are well known and work most reliably as long as the energy in the source is sufficient.
The electric energy sources of such small volume timepieces are essentially cells. As the available energy of the cell diminishes some of its parameters change, in particular the voltage across the cell's terminals drops. In high energy density cells, such as mercury, silver or lithium cells used in timepieces, the voltage drops very rapidly when the cell nears exhaustion. Now, it is essential for the cell's voltage to be sufficiently high if the timepiece and in particular its motor are to work properly. When the voltage drops below a critical threshold, the timepiece starts showing signs of failure and, a few days later, it stops altogether. The first signs of failure are erratic dwells of the motor's rotor in response to drive pulses, whereas other elements of the timepiece, such as the electronic circuits, that are less sensitive to a voltage drop, carry on working normally until the motor comes to a full stop.
Uncertainty about the imminent end of a cell's life is a major drawback that has considerably slowed down sales of early electronic timepieces.
To overcome this drawback it has been proposed to incorporate in electronic timepieces means for detecting the end of a cell's life. These detector means, in known arrangements, consist of an electronic circuit that accurately measures the cell's voltage and of an electronic circuit that produces a warning signal when the voltage reaches the critical threshold, to make the user aware of the imminent stoppage of the motor.
A voltage measurement circuit suitable for use in a watch is for instance described and illustrated in U.S. Pat. No. 4,024,415 wherein terminal X of FIGS. 8 and 9 changes its logic state when the cell's voltage, V.sub.DD, drops below the critical threshold.
This information is used to produce the warning signal. The latter may, for instance, take the form of a periodic modification of the rhythm at which the seconds hand moves forward, causing the needle's motion to become irregular without however affecting the right time or increasing the motor's energy consumption. Such a periodic change may be achieved with a circuit such as described in Swiss Patent Specification No. 607 603.
Since the electronic circuits can still safely operate when the cell's voltage is no longer sufficient to drive the motor, this kind of detecting means would be satisfactory if the voltage measurement circuit did not involve major production difficulties as is the case.
The voltage in the vicinity of the critical threshold needs to be measured most accurately, to within a few tens of millivolts. This accuracy must be influenced neither by temperature nor by the ageing of the elements. Now, such standards are very difficult to achieve even with circuits specially designed for the purpose and often requiring external components having to be individually adjusted, thereby complicating manufacture and increasing costs.
Further, such detecting means will only work properly with the type of cell it has been designed for, e.g. a mercury cell, and if the cell is replaced with a silver cell having a different critical threshold, the detecting means will provide an erroneous indication. Nowadays, however, it has become the practice to produce timepieces able to work equally well with mercury, silver or lithium cells. No known means for detecting the end of a cell's life can therefore be used in this type of timepiece.